The present invention relates generally to fluid-operated cylinder-piston units, which can be used for example for fluid-operated tools.
The present invention also relates to a fluid-operated tool provided with the inventive fluid-operated cylinder-piston unit.
Fluid operated units, in particular hydraulic pumps have usually two states. The first stage is a high flow, low pressure stage, while the second stage is a low flow, high pressure stage. 10,000 psi pumps usually switch at 1,000 psi and the flow decreases at that point to just 10% of the low pressure stage.
The existing fluid-operated cylinder-piston units have the problem that there are only two ways known to increase the speed of the advance stroke. One way to solve this problem is to increase the flow of the pump which, however, requires a more expensive and a larger and heavier pump. Another way is to use the fluid medium from the front piston chamber which contains the piston rod, and let it flow into the rear chamber when the fluid pressure is applied to it. The latter approach has the disadvantages that while the advance stroke speed is increased by the amount of fluid located in the front chamber, the force output is decreased by the same ratio. Also, a switch has to be mounted to the tool in order to switch back the cylinder-piston unit to a regular function manually, to obtain full force based on the piston displacement (usually the bore) and pressure.
The biggest problem is, however, not the advance stroke speed, but the return stroke speed which was not addressed in the above described solutions. Obviously, it takes less fluid medium to return the piston than to advance it, since the return displacement is decreased by the piston rod area. On larger cylinder-piston units, the rear chamber contains a lot of fluid medium which needs to be pressed through a relatively small hole, namely the inlet hole. On hydraulic tools, this hole is usually 1/4 inch. The reason is that more small tools are purchased by customers and since the customer wants to operate all tools off the same pump and the same hoses, the connectors have to stay the same. This represents a restriction. The pump pressure decreases therefore and in many cases the pump goes from its first stage to its second stage to return the piston where its fluid flow speed decreases by 90%. While this is not important at higher force output, it decreases the potential run-down speed at lower force output where most of the turning is done by hydraulic wrenches.